The present invention relates to the art of high voltage control circuits. It finds particular application in conjunction with x-ray tube control circuits and will be described with particular reference thereto.
In x-ray diagnostic equipment, an x-ray tube is commonly turned on or pulsed for a selected duration. More specifically, power is selectively supplied to a high voltage transformer for the selectable duration. High voltage on the secondary side of the transformer is rectified, filtered with a capacitance, and applied across the x-ray tube.
At the end of the actuation period when the supply of electrical potential to the high voltage transformer is terminated, there is still a large amount of electrical energy stored in the capacitance components of the power supply. This energy maintains a potential across the x-ray tube which decays generally exponentially. During this exponential decay period, the x-ray tube produces a generally corresponding decaying amount of x-ray energy. The higher energy portion of the supplied x-rays penetrate the patient and overexpose the photographic film or are detected by electronic x-ray detection circuitry. The lower energy x-rays are absorbed by the patient. Thus, much of the x-ray energy produced after the supply of power to the high voltage transformer has been terminated puts x-rays into the patient with no or detrimental diagnostic value.
In pulsed fluoroscopy experiments, the x-ray tube is pulsed at 0.5 to 5 millisecond intervals to generate relatively low energy x-rays. The stored electrical energy in the system takes a long time, relative to the 0.5 to 5 millisecond pulse intervals to be dissipated. The low energy x-rays from dissipating the capacitors mimics the pulsed low energy pulses and interferes with the diagnostic value of the resultant images.
One prior technique for eliminating the continuing supply of x-ray energy after the selected pulse is terminated is to manufacture the x-ray tube with a grid. By applying appropriate biasing pulses to the grid, the production of x-rays can be sharply turned on and off at the tube. However, such grid-type x-ray tubes require a third control line for which no provision is made in existing equipment. In addition to the incompatibility with existing equipment, grid-type x-ray tubes are limited to operate at lower kV potentials than non-grid tubes.
Another prior art technique is to incorporate a vacuum tube switch in the power supply. At the end of the selected x-ray pulse duration, the vacuum tube is switched conductive providing a low impedance path to discharge the high voltage energy stored in the circuittto ground. However, because x-ray operating voltages are typically on the order of 150 kilovolts, the vacuum tube switch must be physically large. Moreover, such a large vacuum tube generates a large amount of heat for which cooling systems must be provided. Typically, the addition of the vacuum tube and increased cooling capacity approximately doubles the physical size of the power supply circuit. Such a large increase in the size of the power supply renders it unsuitable for use in existing x-ray equipment and increases the complexity of newly designed equipment.
Another solution was to connect a solid state switch, particularly a high voltage triac, between the high potential mains and ground. However, the operating voltage of an x-ray tube exceeds the maximum operating voltage of even high voltage triacs by a large amount. A large array of high voltage triacs, on the order of 100 high voltage triacs, must be ganged together in order to operate at these high potentials, increasing cost and complexity and decreasing reliability. Moreover, an array of 100 high voltage triacs and associated support and biasing circuitry again have a physical size which approximates the physical size of a conventional x-ray tube power supply. Thus, even using solid state switching devices does not significantly decrease the size of the power supply relative to a power supply with a high voltage pentode or other vacuum tube.
The present invention provides a new and improved discharge system which can be added to existing power supplies with a minimal or no increase in their physical size.